1. Field of the Invention
The present invention relates to a photoelectric conversion device and, more particularly, to a photoelectric sensor which is used in an automatic focusing system which focuses an optical system of a photographing or other devices at any desired point in a photographing or imaging frame.
2. Related Background Art
An automatic focus detection system has been proposed which enables automatic focusing at a plurality of points or any desired point within a photographing frame. This system can be realized either by a so-called contrast detection type method or a so-called two-image phase differential detection system. Anyway, in a passive-type system which detects an in-focus state by a computation on the basis of a measured light quantity distribution of a subject, it is necessary to employ a sensor array for determining the above-mentioned light quantity distribution.
An automatic focusing system for detecting focus on a plurality of points by utilizing a passive technique has been already realized in the field of single-lens reflex cameras. Such a system employs a linear sensor array for each of a finite number of focus detection points. In order to reduce the production cost, these sensor arrays are integrated on a single chip. Practical arrangements of such sensor arrays are disclosed, for example, in Japanese Patent Laid-Open Application Nos. 63-11906, 63-172209 and 1-271716.
A known focus detection optical system will be described with specific reference to FIGS. 1A and 1B. This known optical system has a field lens 20, a multi-slit field mask 21, a secondary image-forming lens 22 composed of a pair of positive lenses arranged side by side, and a sensor device 23 having a plurality of pairs of photoelectric sensor arrays. The multi-slit field mask 21 is positioned in the vicinity of an expected imaging plane of an objective lens (not shown), and has slits 21a, 21b and 21c which determine respective range-finding fields. The secondary image forming lens 22 focuses, for example, a portion of the subject image defined by the slit 21a substantially on the sensor array pair 23a, 23b.
Likewise, the subject image portions defined by the slits 21b and 21c are focused substantially on the sensor array pairs 23c, 23d and 23e, 23f, respectively. Pieces of information concerning light quantities, received by the respective sensor arrays, are read as electrical signals and are subjected to a correlation computation so that a value indicative of state of focusing of the object lens on the subject image in the range-finding field, defined by each slit, is determined. The range-finding fields defined by the three slits can be set in an imaging frame 27 at 29L, 29C and 29R, as shown in FIG. 1B.
When only few focus detecting position are used as in the described system, it is possible to provide linear sensor arrays corresponding to the respective focus detecting positions, on a single chip in a discrete manner, as is the case of the sensor device 23. In such a case, a device of high scale of integration can be obtained because the regions between adjacent sensor arrays can be used as the spaces for installing logic and analog circuits which would support functions of the sensor arrays.
This way of integration, however, cannot suitably be used when a greater number of focus detection points are arranged more densely. This is because each linear sensor array has to be assisted by various additional circuits other than sensor pixels. For instance, it is necessary to provide, for each of the sensor arrays, a temporary accumulation means for serially outputting photo-charge information which has been stored in a batch manner, a serial information transfer system, a shift register for performing clocking to enable the information to be read sequentially, and so forth. In general, such additional circuits require much greater installation area than the sensor arrays. As a consequence, the number of the sensor arrays which can be constructed in one chip is strictly limited.
In order to attain a greater number of sensor arrays, i.e., a higher density of sensor arrays, it is preferred to use a so-called area sensor in which sensor cells are regularly arranged two-dimensionally. State of focusing on a specific subject position can be detected by computing information from pixels in a suitable portion of the light-receiving area in the area sensor. Focus detection relying upon an area sensor has already been put to use in the fields of TV cameras and VTR-integrated cameras, which employ electronic imaging devices, because the imaging device employing an area sensor can also serve as the focus detection sensor. In such a case, however, the use of an area sensor is not intended for multi-focus detection but is employed for the purpose of reducing the cost of the apparatus.
FIG. 2 shows an example of the optical apparatus having an area sensor array which serves both as imaging device and a focusing sensor. A focus lens 1 is adapted to be driven by a focus motor 2. A solid-state imaging device 4 is attached to the center of the bimorph 3. The solid-state imaging device 4 performs photoelectric conversion of optical image information formed by the focus lens 1, and has a large number of pixels, typically 100,000 to 500,000 pixels. Electrical signals thus obtained are supplied to a video-signal processing system (not shown) which produce video signals. A bimorph 3 is driven by A.C. power from a bimorph driving circuit 8 and is adapted to oscillate the solid-state imaging device 4 in the direction of the optical axis. The output signal from the solid-state imaging device 4 is delivered to a defocus detection circuit 10 which detects, during the axial oscillation of the solid-state imaging device 4, whether the focus is in front of or behind the subject, and produces a signal to activate the focus motor 2 so as to move the focus lens 1 in such a direction as to reduce the amount of defocus.
In general, a photographing frame contains the major subject to be photographed and background. It is, therefore, necessary to define, by suitable measure, the area to be picked up by the defocus detection circuit out of the entire area of the imaging frame. Conventionally, the area or portion to be used as the object of the focus detection is defined in a central region of the imaging frame. Alternatively, a framework of a predetermined size is set to surround the center of the imaging frame and the focus detection is conducted on a point which exhibits the highest contrast within the area defined by the framework.
Known focus detection systems employing area sensors composed of two-dimensionally arranged pixels, however, suffer from various drawbacks or shortcomings as stated below, so that they can not satisfactorily perform a high degree of function for comparing and evaluating states of focusing on a multiplicity of focus detection points in a frame.
Firstly, it is to be understood that an ordinary imaging area sensor does not have any means for enabling a random access to information concerning local portions of the subject image in the imaging frame. Multi-focus detection requires that pieces of image information derived from different focus detection points are quickly calculated for the purpose of comparison and evaluation, and the results of the computation are used in the focus control operation. The above-mentioned computation is performed by hardware based upon a microprocessor or a digital circuit such as a DSP and, therefore, essentially requires an A/D converter, for converting the image information, and a digital memory in which the converted data is to be stored. Data sampling from plural focus detection points is greatly facilitated in terms of the system hardware structure, memory capacity, speed of A/D converter, and so forth, when suitable means are provided for enabling a random access to the data available from these focus detection points. Unfortunately, however, a conventional area sensor does not have a function for reading data from designated focus detection blocks in a random manner. It is impossible to realize a focus detection system with such area sensors. This problem is serious particularly in the focus detection system of the phase differential type which essentially requires photoelectric outputs of two optical images which are formed from the same subject through different optical paths. To this end, it is necessary to employ a sensor device which can control a pair of discrete image blocks in synchronization.
In ordinary area sensors, the data on the entire area of the sensor are read at a constant high clock speed and the required data is picked up during the reading. In addition, the timing of the reading is undesirably limited due to hardware. As a consequence, an impractically long time is required for the system to obtain the result. Thus, the known area sensors cannot provide satisfactory performance. In addition, the known area sensor, which has no function for enabling a random access to local portions of the image area, cannot comply with a significant demand by the photographer who often wishes to change, in a random manner, the point at which the lens is to be focused, depending on factors such as the focal point of the photographing lens or the type of the subject.
Secondly, it is to be pointed out that the known area sensor does not have a function for optimizing the signal charge accumulation on each of a plurality of focus detection points which usually have different levels of luminance and contrast. In general, persons, sceneries and other photographing or video-imaging subjects have wide variations of light quantity. Namely, light quantity largely vary according to portions of such subject. The portion of the photographer's interest is not always the portion which has the highest luminance. For instance, it is often experienced that the luminance of background is 10 to 10.sup.2 greater than that of the face of a person as the subject. When a reflection of sunlight exists in the background, the luminance level of the background may be 10.sup.3 greater than that of the major subject. Therefore, when an area sensor is used in a focus detection system having a multiplicity of focus detection points, it is necessary that the control of the accumulation of signal charges and the gain of the amplifier for reading are optimumly controlled for each of different points of focus detection. This requirement is not met by ordinary silicon photoelectric sensors which are used at normal temperatures and which have a dynamic range as small as 10.sup.2 to 10.sup.3. Namely, it is impossible to conduct control over the entire imaging area in response to the wide variation in the luminance, while maintaining required levels of S/N ratio on all the points of focus detection. Namely, when the known area sensor is used, the photographing lens is focused on a point which has the highest level of luminance. Consequently, a portion with a high level of luminance or contrast is preferentially focused regardless of the photographer's intention.
It is also to be pointed out that a higher density in the arrangement of the focus detection points essentially requires a reduction in the pixel size in each photoelectric sensor. As a consequence, the quantity of light distributed to each image block for focus detection is reduced to affect the focus detection performance particularly in the low-luminance range. Thus, the lower limit of luminance for focus detection, as well as effective reach of any auxiliary illuminating means, is reduced when the known area sensor is used in a multi-point focus detection system.